Histological features of ichthyosaur humeral sections.

A–C and D–E, Ichthyosaurus SMNS Unnumbered outer and inner parts of the section respectively. A, primary fibrolamellar bone (FLB) in natural light (NL); note the isotropic nature of the primary fibrous bone (FB); B, FLB in polarized light (PL) illustrating the variable orientations of the primary osteons; C, ‘unusual parallel-fibered bone’ (UPFB) in PL with gypsum filter; D–E, extremely compact core of the section made of almost exclusively secondary bone in PL and PL with gypsum filter. F–I, Ichthyosaurus IPB R222 outer cortex. J, Ichthyosaurus SMNS Unnumbered outer cortex. F–H, primary FLB in PL with gypsum filter; F, laminar organization; G, radiating FLB; H, important amount of primary FB in the osseous trabeculae. I–J, UPFB in I, PL with gypsum filter and J, NL respectively; note the occurrence of simple vascular canals. FB: fibrous bone; PFB: parallel-fibered bone.